In order to measure, in a fiber ribbon welding device, various geometrical quantities such as fiber gap, fiber diameter, offset laterally between the exterior sides of the fibers and between the cores of the fibers, etc., of the fibers to be welded to each other the positions of the fibers in the xyz-coordinate system of the device must be determined. In order to make it the ends of the fibers must be observed in two different directions or equivalently two pictures must be taken in two such directions. These directions must then be placed obliquely in relation to a plane through the end portions of the fiber ribbons, most preferably in directions of about 45.degree. to this plane, so that the two directions have an angle of about 90.degree. to each other, in order to obtain the largest possible information on the positions of the fibers. In the imaging operation then thus the very object, i.e. the end portions of the fiber ribbons, is located obliquely, having some parts located more distant from and other parts at a shorter distance from the opening of a lens system which is included in the camera, by means of which such a picture is usually made. It causes in turn a varying magnification in the picture captured by the camera and thereby a varying light intensity in the picture. The varying light intensity in the picture is particularly embarrassing as to the generally used light background, which gives difficulties when measuring on all of the picture field and when adjusting the camera in order to obtain a maximal sensitivity in regard of the determination of the position of the outlines of the ends of the different fibers in relation to each other in one single picture.
Systems for achieving a more uniform incoming light intensity to different devices are known from for example U.S. Pat. Nos. 5,442,414 and 4,391,521 and from the published British patent application No. 2 226 145.